The present invention relates to a method for measuring the concentration of water in argon, hydrogen, nitrogen and helium by ionization mobility spectrometry.
These gases are widely used in the semiconductor industry as transport gases in which reactive species are diluted or as support gases for plasma formation in cathodic deposition processes (in particular, helium and argon are employed for these uses), as well as real reagents in the process (in particular in the case of nitrogen). Among these gases, argon is the most important for industry. In the rest of this specification this gas will be mainly referred to, but the invention may also be applied with the same results to the other cited gases.
The pureness of the argon employed in the semiconductor industry is particularly important. As a matter of fact, contaminants which may he present in the reagents or in the reaction environment can be incorporated into the solid state devices, thereby altering their electrical or magnetic properties and thus leading to production rejects.
Argon purification is the subject-matter of various patents, such as British Patent GB-B2177079 (similarly, British Patent GB-B-2177080 relates to nitrogen purification and U.S. Pat. Nos. 5,558,844 and 5,556,603 relate to hydrogen purification). According to this patent, argon is purified by passing it through a bed made of a getter material (an alloy based on zirconium, vanadium and iron) kept at a temperature between 350 and 450° C. Alternatively, purifiers working at room temperature are commonly employed, which are based on the use of nickel generally dispersed onto a high surface area support, such as alumina or molecular sieves. With these methods the impurity content can be reduced below one part per billion (ppb, equivalent to one impurity molecule for every 109 molecules of argon).
Under these conditions it is also necessary to allow for control of the gas purity and its constancy over time, for detecting increments of the impurity concentration, due for example to working anomalies of the purifiers, tightness losses of the gas lines or other reasons.
A particularly interesting technique for carrying out this analysis is ionization mobility spectrometry, also referred to in the field with the initials IMS (the same initials are also used for the instrument carrying out this technique, i.e., “Ionization Mobility Spectrometer”). Interest in this technique derives from its very high sensitivity, combined with the limited size and costs of the instrument. By operating under suitable conditions species in the gas or vapor phase can be detected in a gas medium in quantities of picograms (pg, i.e., 10−12 grams), or in concentrations of parts per trillion (ppt, equivalent to a molecule of the analyzed substance for every 1012 molecules of the gas sample). IMS instruments and analysis methods in which these are employed are disclosed, for example, in U.S. Pat. Nos. 5,457,316 and 5,955,886 in the name of the US company PCP Inc.
An IMS instrument is essentially made up of a reaction zone, a separation zone and a charged particles detector.
In the reaction zone the ionization of the sample comprising the gases or vapors to be analyzed in a transport gas takes place, commonly by means of beta-radiation emitted by 63Ni. The ionization takes place mainly on the transport gas with the formation of the so-called “reagent ions,” whose charge is then distributed on the species present according to their electronic or proton affinities or to their ionization potentials. The book “Ion Mobility Spectrometry” by G. A. Eiceman and Z. Karpas, published in 1994 by CRC Press, can be referred to for an illustration of the (rather complex) charge transfer principles which are the basis of the ionization mobility spectrometry technique.
The reaction zone is divided from the separation zone by a grid which, kept at a suitable voltage, prevents the ions produced in the reaction zone from entering into the separation zone. The moment at which the grid voltage is turned off, thus allowing the ions to enter the separation zone, is the “time zero” of the analysis. The separation zone comprises a series of electrodes, which create an electric field such that the ions are carried from the reaction zone toward the detector. This zone is kept at atmospheric pressure. Therefore, the motion speed of the ions depends on the electric field and on the cross-section thereof in the gaseous medium. By recording the current reading of the particle detector according to the time elapsed from “time zero,” peaks corresponding to the so-called “drift time” of the different present ions are obtained. By determining the drift time it is possible to go back to the presence of the substances which are the object of the analysis.
In spite of its conceptual simplicity, the application of the technique involves some difficulties in the interpretation of the analysis results.
The instrument, analogously to chromatographs, provides as a result of the analysis the crossing times (drift times in the case of the IMS) of the ions corresponding to the species present, but it does not provide indications about the chemical nature of the ion corresponding to each peak.
For attributing each peak to a specific ion, the IMS may be connected to a mass spectrometer, which determines the chemical nature of each ion, but in this way the above mentioned advantages of low cost and compactness are renounced.
Alternatively, it is possible to resort to calibration tests, wherein a sample formed of an extremely pure transport gas containing the substance which is the object of the analysis is used, and the drift time of this latter gas is determined. However, the analysis under real conditions is complicated, since the various ionic species which are present may lead to phenomena of charge transfer with each other or with neutral molecules present, so that the determined drift times can be the characteristic times of species different from those whose presence is to be determined.
A possible method for overcoming the problems found in the real analyses consists in adding the sample gas with a specific substance, called a “doping gas,” which, according to various mechanisms, obtains the effect of significantly decreasing the sensitivity of the measurement toward the species differing from the one which is the object of the analysis. As examples of practical application of the method of the doping gas may be mentioned U.S. Pat. No. 4,551,624, relating to the addition of ketones or halogenated gases to the gas to be analyzed; U.S. Pat. Nos. 5,032,721 and 5,095,206, relating, respectively, to the use of phenols and sulfur dioxide in the analysis of acid gases; and U.S. Pat. No. 5,238,199, relating to the use of amines in the analysis of chlorine dioxide. However, the doping gas method disadvantageously requires that a tank of this gas and means for its dosage in the gaseous medium are added to the system, thus leading to a complication of the measuring system based on the IMS instrument.
In the methods not based on the employment of a doping gas the possibility of carrying out a quantitative analysis is bound to the presence of a reactant ion. As previously described, the reactant ion generally is an ion corresponding to the gas present in higher amount in the mixture. Then, the reactant ion formed in the ionization zone transfers the charge to the other species present according to complex chemical balances. When the concentration of impurities increases, the charge quantity transferred thereto from the reactant ion also increases, until the latter is extinguished. In the IMS spectrum this mechanism is reflected by the intensity increase of the peaks related to the impurities and by the simultaneous intensity decrease of the peak of the reactant ion, commonly defined in the field as “Reactant Ion Peak” or RIP, up to its extinction. Obviously, when this condition is reached, the concentration of the ions relating to the impurities and the intensity of the relevant peaks in the spectrum do not grow any more, even if the effective impurity concentration increases and therefore it is no longer possible to carry out a quantitative IMS analysis in this way. In the presence of water in argon, the RIP is extinguished with concentrations of about 10-15 ppb. Thus, according to the state of the art, this value is the maximum measurement limit of this impurity in argon with the IMS technique.